Replace pythonz with pyenv

Depending on Python to manage Python is silly.
Add utility aliases for Cygwin

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
Remove duplicate alias definitions
Simplify a conditional
Merge branch 'master' of https://github.com/sorin-ionescu/prezto

Conflicts:
	modules/python/init.zsh